Nucleoside analogues became increasingly important in various genetic studies and in genetic engineering. These compounds can be inserted into oligonucleotide sequences thereby proving the structure and functions of different parts of such nucleic acids, and have also the potential of being used as complex vectors that may eventually include target-drugs. The target-nucleosides thus engineered bind the nucleic acid sequence containing a mutated sequence associated with a particular disease or disorder, and the drug efficiency is thus dramatically increased while side effects are similarly reduced.
Various classes of nucleoside analogues have been reported, and among those having similar structures and H-bonding capabilities, the C-nucleosides attracted attention due to the increased stability of the glycoside bond towards enzymes and in acidic conditions.
A broad range of C-nucleosides with bicyclic, heterocyclic purine-mimics moieties have been recently reviewed (Wellington and Benner, 2006; Stambasky et al., 2009). Particular such C-nucleosides are pyrazolo[1,5a]-1,3,5-triazine derivatives, which are structurally related to purines and have similar biophysical and chemical properties.
A particular C-nucleoside discussed in the scientific literature is 4-amino-8-(β-D-ribofuranosyl)pyrazolo[1,5-a]-1,3,5-triazine. Tam et al. (1976) show the syntheses of 4-amino-8-(β-D-ribofuranosyl)pyrazolo[1,5-a]-1,3,5-triazine and 4-oxo-3H-8-(β-D-ribofuranosyl)pyrazolo-[1,5-a]-1,3,5-triazine, identified therein as compounds 10 and 16, respectively, and disclose that compound 10 has a moderate inhibitory activity against various mouse leukemia cells, and it is more active than formycin in all systems tested. Tam et al. (1979) describe the synthesis of certain C-7 ribosylated pyrazolo[1,5-a]-1,3,5-triazine C-nucleosides starting from the aforesaid 4-amino-8-(β-D-ribofuranosyl)pyrazolo[1,5-a]-1,3,5-triazine; however, do not show any biological activity of the compounds prepared. Miller et al. (1979) disclose kinetic constants for substrates and inhibitors of highly purified rabbit liver adenosine kinase for dozens of nucleosides and nucleoside analogues including 4-amino-8-(β-D-ribofuranosyl)pyrazolo[1,5-a]-1,3,5-triazine, identified therein as compound 94. Otter and Klein (1996) describe NMR study of various purine-like C-nucleosides including the pyrazolotriazolyl nucleoside analogues 4-amino-8-(β-D-ribofuranosyl) pyrazolo[1,5-a]-1,3,5-triazine and 4-methylthio-8-(β-D-ribofuranosyl)pyrazolo[1,5-a]-1,3,5-triazine, identified therein as compounds 4 and 5, respectively; however, do not show any biological activity of the compounds prepared. Liang et al (1997) describe a process for the synthesis of, inter alia, 4-amino-8-(β-L-ribofuranosyl)pyrazolo[1,5-a]-1,3,5-triazine, and disclose that no significant anti-HBV and anti-HIV activities were observed for this compound. WO 2002032920 discloses compositions and methods for treatment of a Flaviviridae, Orthomyxoviridae or Paramyxoviridae infection, or conditions related to abnormal cellular proliferation, using certain nucleoside analogues including pyrazol-triazine based nucleosides, in particular, 4-amino-8-(β-D-ribofuranosyl)pyrazolo[1,5-a]-1,3,5-triazine. Zarubin et al. (2002) disclose a theoretical study of antagonists and inhibitors of mammalian adenosine deaminase (ADA), in particular, certain isomeric aza-deaza analogues of adenosine and their N1-protonated forms, including the compound 4-amino-8-(β-D-ribofuranosyl)pyrazolo[1,5-a]-1,3,5-triazine, identified therein as z5c9Ado, and its 1-N protonated form, identified therein as 1H+-z5c9Ado. Zarubin et al. (2003) describe computer modeling studies to find relationships between the molecular structure of certain isosteric analogues of adenosine, including their N1-protonated forms (except for that of 1-deaza analogues), and substrate properties for the mammalian ADA. This reference refers, inter alia, to D-ribitol, 1-C-(4-aminopyrazolo[1,5-a]-1,3,5-triazin-8-yl)-1,4-anhydro-,(1S), which is a chiral isomer of the aforementioned 4-amino-8-Q3-D-ribofuranosyl) pyrazolo[1,5-a]-1,3,5-triazine. Il'icheva et al. (2005) disclose conformational models of the active site of ADA and its complexes in the basic state with adenosine and various isosteric analogues of the aza, deaza, and azadeaza series, including the aforementioned z5c9Ado. WO 2007002191 discloses a method for the preparation of 9-deazapurine derivatives, including (2S,3R,4S,5R)-2-(4-aminopyrazolo[1,5-a][1,3,5]triazin-8-yl)-5-(hydroxymethyl)-tetrahydrofuran-3,4-diol and (2S,3R,4R,5R)-2-(4-aminopyrazolo[1,5-a][1,3,5]triazin-8-yl)-5-(hydroxyl methyl)-3-methyl-tetrahydrofuran-3,4-diol, identified therein as compounds 9-3 and 9-3′, respectively; however, does not show any biological activity of these compounds.
WO 2003093290 discloses nucleosides derivatives in which each one of the carbon atoms at positions 2′ and 3′ of the ribose moiety is substituted with a hydroxyl group, but at least one of said carbon atoms is further substituted with a group different than hydrogen. Examples of such nucleosides include, inter alia, 8-(2′-C-methyl-β-D-ribofuranosyl)-pyrazolo[1,5-a][1,3,5]triazin-4-ylamine, 8-(2′-C-methyl-(3-D-ribofuranosyl)-3H-pyrazolo[1,5-a]-[1,3,5]triazin-4-one, and 2-amino-8-(2′-C-methyl-β-D-ribofuranosyl)-3H-pyrazolo-[1,5a][1,3,5]triazin-4-one, identified therein as compounds 98-100, respectively. According to this publication, these compounds are useful in treatment of hepatitis C virus infections.
The syntheses of several deoxyadenosine C-nucleoside analogues having a pyrazolotriazine heterocycle as a base have also been reported. Raboisson et al. (2002) disclose large-scale preparation of 2′-deoxy-C-nucleosides of pyrazolo[1,5-a]-1,3,5-triazines, such as 8-(2′-deoxy-β-D-ribofuranosyl)-2-methyl-4-(N-methyl amino)pyrazolo[1,5-a]-1,3,5-triazine-3′,5′-bisphosphate; 8-(2′-deoxy-β-D-ribo furanosyl)-2-methyl-4-(N-methyl-N-phenylamino)-pyrazolo[1,5-a]-1,3,5-triazine-3′,5′-bisphosphate; and 8-(2′-deoxy-β-D-ribofuranosyl)-2-methyl-4-(N-methyl amino)pyrazolo[1,5-a]-1,3,5-triazine, identified therein as compounds 2, 20 and 21, respectively; and further disclose that compound 2 strongly inhibits ADP-induced human platelet aggregation and shape change. Raboisson et al. (2008) disclose cyclic nucleotide phosphodiesterase type 4 inhibitors, in particular, pyrazolo[1,5-a]-1,3,5-triazine ring systems as adenine bioisosteres, including pyrazolo[1,5-a]-1,3,5-triazine compounds substituted at position 8, and that some of those compounds strongly inhibit lipopolysaccharide (LPS)-induced TNFα release from human mononuclear cells from healthy subjects. Mathieu et al. (2006) show stereoselective synthesis of 3′-substituted 2′-deoxy C-nucleoside pyrazolo[1,5-a]-1,3,5-triazines and their 5′-phosphate nucleotides including 8-(2′-deoxy-β-D-ribofuranosyl)-2-methyl-4-(N-methylamino)pyrazolo[1,5-a]-1,3,5-triazine-3′,5′-bisphosphate, identified therein as compound 1, and certain analogues thereof.
WO 9316094 discloses structural analogues of the six N-nucleosides commonly found in RNA and DNA, which are said to be inherently fluorescent under physiological conditions, including pyrazolo-triazine based nucleoside analogues; however, exemplifies formycin-A based derivatives only, i.e., pyrazolo-pyrimidine nucleoside analogues. As stated in this publication, such analogues may be incorporated into DNA and/or RNA oligonucleotides to produce fluorescent oligonucleotides having prescribed sequences, which may be used for detection assays.
C-nucleosides can be synthesized using different strategies such as building up of an aglycon on a carbohydrate residue or vice versa; modifying existing C-nucleosides; and direct coupling of a suitable preformed heterocyclic aglycon with the carbohydrate unit (Stambasky et al., 2009). The latter is of great interest since it consists of a direct C—C attachment of the preformed base analogue to the deoxyribose unit in a highly stereo and regioselective manner using the Heck cross-coupling reaction initially reported by Daves (Arai and Daves, 1978; Daves, 1990). Thus, when an iodopyrazolotriazine aglycon is reacted with a protected glycal using the Heck reaction, the new C-glycosidic bond is formed selectively at the anomeric carbon, despite the presence of a rich-electron double-bond. The initial attack occurs from the less hindered face of the glycal ring, and this regioselective effect may be enhanced when a bulky protective group is present in the 3-position, in combination with a bulky Pd-catalyst ligand (Zhang et al., 1995). A few examples of unexpected exclusive β-anomer formation using a fully unprotected glycal have also been described (Raboisson et al., 2002; Zhang et al., 1995).